Liquid dispenser systems, such as liquid soap and sanitizer dispensers, provide a user with a predetermined amount of liquid upon actuation of the dispenser. In addition, it is sometimes desirable to dispense the liquid in the form of foam by, for example, injecting air into the liquid to create a foamy mixture of liquid and air bubbles by use of an air pump or air compressor. Most foam pumps have a constant volume output and to change the volume requires one to change the pump or “short stroke” the pump. A foam pump is short stroked when a user rapidly pushes a dispense actuator and the pump does not have time to move back to it rest position, or the dispenser or a user prevents the actuator from returning to its full stroke before actuating the actuator an additional time. Problems often occur with foam pumps when they are short stroked. If a blocking plate is added to the dispenser actuator so that the actuator does not drive the liquid piston to its full length, many pumps will not prime because an air bubble remains in the liquid piston. Another problem is that air trapped in the liquid piston results in an inconsistent output.